I thin Nunn folds. GGG ramps things up in the late rounds and Nunn breaks. He has the tools to win, but I don't think he has the chin.
I know there's nothing wrong with his chin. But there's some difference between having a decent chin, and having one good enough to not get KOed by GGG. If James Toney had enough power to do it, so does Golovkin IMO.